Low-boiling hydrocarbons, such as methane, ethane, propane, butane and iso-butane, are normally present in conduits which are used for the transport and processing of natural gas and crude oil. When varying amounts of water are also present in such conduits the water/hydrocarbon mixture is, under conditions of low temperature and elevated pressure, capable to form gas hydrate crystals. Gas hydrates are clathrates (inclusion compounds) in which small hydrocarbon molecules are trapped in a lattice consisting of water molecules. As the maximum temperature at which gas hydrates can be formed strongly depends on the pressure of the system, hydrates are markedly different from ice.
The structure of the gas hydrates depends on the type of the gas forming the structure:methane and ethane form cubic lattices having a lattice constant of 1.2 nm (normally referred to as structure I) whereas propane and butane form cubic lattices having a lattice constant of 1.73 nm (normally referred to as structure II). It is known that even the presence of a small amount of propane in a mixture of low-boiling hydrocarbons will result in the formation of type II gas hydrates which type is therefore normally encountered during the production of oil and gas. It is also known that compounds like methyl cyclopentane, benzene and toluene are susceptible of forming hydrate crystals under appropriate conditions, for example in the presence of methane. Such hydrates are referred to as having structure H.
Gas hydrate crystals which grow inside a conduit such as a pipeline are known to be able to block or even damage the conduit. In order to cope with this undesired phenomenon, a number of remedies has been proposed in the past such as removal of free water, maintaining elevated temperatures and/or reduced pressures or the addition of chemicals such as melting point depressants (antifreezes). Melting point depressants, typical examples of which are methanol and various glycols, often have to be added in substantial amounts, typically in the order of several tens of percent by weight of the water present, in order to be effective. This is disadvantageous with respect to costs of the materials, their storage facilities and their recovery which is rather expensive.
Another approach to keep the fluids in the conduits flowing is taken by adding crystal growth inhibitors and/or compounds which are in principle capable of preventing agglomeration of hydrate crystals. Compared to the amounts of antifreeze required, already small amounts of such compounds are normally effective in preventing the blockage of a conduit by hydrates. The principles of interfering with crystal growth and/or agglomeration are known.
Several classes of compounds have been proposed as potential crystal growth inhibitors. For instance, cold-water fish peptides and glycopeptides appear to be effective in interfering with the growth of gas-hydrate crystals but their production and use for this purpose are rather uneconomical. The use of polymers having a linear backbone such as the (co-)polymers N-vinyl-2-pyrrolidone for inhibiting the formation, growth and/or agglomeration of gas hydrates has been described in International Patent Application Publication WO93/25798. The use of compounds normally referred to as “quats” has been described in, inter alia, EP-A-736130, EP-A-824631, U.S. Pat. No. 5,648,575 and WO 98/05745. The “quat” type compounds focus around quaternary onium, in particular quaternary ammonium, compounds containing two or three lower alkyl chains, preferably containing C4 and/or C5 alkyl groups and one or two longer alkyl chains, preferably containing at least eight carbon atoms, which are bound to the central nitrogen moiety, thus forming a cationic species which is matched by a suitable anion such as a halide or other inorganic anion. Preferred “quats” comprise two long chains, comprising between 8 and 50 carbon atoms, which may also contain ester groups and/or branched structures.
It has now been found that a completely different class of compounds can also be used in the combat against hydrate blockage of conduits thus substantially widening the window of application in this area.